The chief content of the Cartesian system may be seen condensed in the following epitome.
1. If science would have any thing fixed and abiding, it must begin with the primal ground of things; every presupposition which we may have cherished from infancy must be abandoned; in a word, we must doubt at every point to which the least uncertainty is attached. We must therefore doubt not only the existence of the objects of sense, since the senses so frequently deceive, but also the truths of mathematics and geometry—for, however evident the proposition may appear that two and three make five, or that the square has four sides, yet we cannot know but what God may have designedly formed us for erroneous judgments. It is therefore advisable to doubt every thing, in fact to deny every thing, to posit every thing as false.
2. But though we posit every thing as false to which the slightest doubt may be attached, yet we cannot deny one thing, viz., the truth that we, who so think, do exist. But rather from the very fact that I posit every thing as false, that I doubt every thing, is it manifest that I, the doubter, exist. Hence the proposition: I think, therefore I am (_cogito ergo sum_), is the first and most certain position which offers itself to every one attempting to philosophize. Upon this the most certain of all propositions, the certainty of all other knowledge depends. The objection of _Gassendi_ that the truth of existence follows from any other activity of man as well as from thinking, that I might just as well say: I go to walk, therefore I exist,—has no weight; for, of all my actions, I can be absolutely certain only of my thinking.
3. From the proposition I think, therefore, I am, the whole nature of the mind may be determined. When we examine who we are who hold every thing to be false that is distinct from ourselves, we see clearly that neither extension nor figure, nor any thing which can be predicated of body, but only thought, belongs to our nature. I am therefore only a thinking being, _i. e._ mind, soul, intelligence, reason. Thought is my substance. Mind can therefore be apprehended clearly and completely for itself alone, without any of those attributes which belong to body. Its conception contains nothing of that which belongs to the conception of body. It is therefore impossible to apprehend it through any sensuous representation, or to make an image of it: it apprehends itself only through the pure intelligence.